Although the officials cautioned that Obama had not made a final decision, all indications suggest that the strike could occur as soon as United Nations inspectors, who are investigating the August 21 attack that killed hundreds of Syrians, leave the country. They are scheduled to depart Damascus on Saturday.

The White House is to present its case for military action against Syria to Congressional leaders on Thursday night, reported the New York Times. Administration officials assert that the intelligence will show that forces loyal to President Bashar Al-Assad carried out the chemical weapons attack in the suburbs of Damascus.

The intelligence does not tie Assad directly to the attack, officials briefed on the presentation said, but the administration believes that it has enough evidence to carry out a limited strike that would deter the Syrian government from using these weapons again.

Obama, officials said, is basing his case for action both on safeguarding international standards against the use of chemical weapons and on the threat to America’s national interests posed by Syria’s use of those weapons.

Administration officials said that threat was both to allies in the region, like Turkey and Israel, and to the United States itself, if Syria’s weapons fell into the wrong hands.

Obama’s rationale for a strike creates a parallel dilemma to the one that President George W. Bush confronted 10 years ago, when he decided to enter into a far broader war with nearly 150,000 American troops in Iraq — one that the Obama administration says differed sharply from its objectives in Syria — without seeking an authorizing resolution in the United Nations.

In that case, the officials said, Bush was seeking to overthrow the Iraqi government. In this one, they argue, Obama is reinforcing an international ban on the use of chemical weapons, and seeking to prevent their use in Syria or against American allies.

Russia and China, Syrian allies and permanent members of the Security Council, have so far refused to support any military action against Assad. But Obama, his aides say, has reached what one called “a pragmatic conclusion” that even the most ironclad evidence that chemical weapons were used would not change Russia’s objections.

“We have been trying to get the UN Security Council to be more assertive on Syria even before this incident,” Benjamin J. Rhodes, the deputy national security adviser for strategic communications, said Thursday in an interview. “The problem is that the Russians won’t vote for any accountability.”

One central piece of the White House intelligence, officials say, is an intercepted telephone call in which a Syrian commander seems to suggest that the chemical attack was more devastating than intended. “It sounds like he thinks this was a small operation that got out of control,” one intelligence official said Thursday.

Rhodes and other aides insist that there are major differences from the decision that faced Bush in 2003. “There is no direct parallel with 2003, given that the United States at that time had to prove the existence of weapons of mass destruction in a country where we were going to do a military intervention aimed at regime change,” Rhodes said, according to the New York Times.

"What we're talking about here is something very discrete and limited," he said.

Obama seemed determined earlier in the week to strike in Syria as a warning to Assad for crossing the red line of using chemical weapons, but on Wednesday the American president told PBS that he had not yet decided whether to carry out such a military strike.